Two communication devices need to perform negotiation of communication capacity of physical layers prior to the communication, the communication capacity including communication rate and communication mode. For example, the communication rate includes 10 Mbps, 100 Mbps, and 1000 Mbps, and the communication mode includes half duplex, full duplex and so on. Through negotiation, rate and mode are agreed, for example, 10 Mbps half duplex, or 100 Mbps full duplex, etc. may be negotiated. The communication capacity negotiation of physical layers is performed between the physical layers of both communication parties.
Throughout the drawings, identical reference numbers designate similar, but not necessarily identical, elements.